Ca cofactor of the water-oxidation complex: Evidence for a Mn/Ca heteronuclear cluster

• DOI: 10.1071/sa0403261
• Publication Date: 2001-07-25
• Authors: Cinco, Roehl M.; Robblee, John H.; Messinger, Johannes; Fernandez, Carmen; McFarlane, Karen L.; Pizarro, Shelly A.; Sauer, Ken; Yachandra, Vittal K.
• Keywords: 59 Basic Biological Sciences; Atoms; Manganese; Basic biological sciences; Electric Fields; Water Mn Complex Ca Cofactor Strontium Photosynthetic Water Oxidation Oxygen Evolution X-Ray Absorption Spectroscopy; Carbon; Absorption; Chemistry; Structural Models; Strontium; S States; Oxidation; Mn Complex Ca Cofactor Strontium Photosynthetic Water Oxidation Oxygen Evolution X-Ray Absorption Spectroscopy; Calcium; Mn complex Ca cofactor strontium photosynthetic water oxidation oxygen evolution X-ray absorption spectroscopy; Photosynthesis

Calcium and chloride are necessary cofactors for the proper function of the oxygen-evolving complex (OEC) of Photosystem II (PS II). Located in the thylakoid membranes of green plants, cyanobacteria and algae, PS II and the OEC catalyze the light-driven oxidation of water into dioxygen (released into the biosphere), protons and electrons for carbon fixation. The actual chemistry of water oxidation is performed by a cluster of four manganese atoms, along with the requisite cofactors Ca{sup 2+} and Cl{sup -}. While the Mn complex has been extensively studied by X-ray absorption techniques, comparatively less is known about the Ca{sup 2+} cofactor. The fewer number of studies on the Ca{sup 2+} cofactor have sometimes relied on substituting the native cofactor with strontium or other metals, and have stirred some debate about the structure of the binding site. past efforts using Mn EXAFS on Sr-substituted PSII are suggestive of a close link between the Mn cluster and Sr, within 3.5 {angstrom}. The most recent published study using Sr EXAFS on similar samples confirms this finding of a 3.5 {angstrom} distance between Mn and Sr. This finding was base3d on a second Fourier peak (R {approx} 3 {angstrom}) in the Sr EXAFS from functionalmore » samples, but is absent from inactive, hydroxylamine-treated PS II. This Fourier peak II was found to fit best to two Mn at 3.5 {angstrom} rather than lighter atoms (carbon). Nevertheless, other experiments have given contrary results. They wanted to extend the technique by using polarized Sr EXAFS on layered Sr-substituted samples, to provide important angle information. Polarized EXAFS involves collecting spectra for different incident angles ({theta}) between the membrane normal of the layered sample and the X-ray electric field vector. Dichroism in the EXAFS can occur, depending on how the particular absorber-backscatterer (A-B) vector is aligned with the electric field. Through analysis of the dichroism, they extract the average number of scatterers per absorbing atom (N{sub iso}). Constraints on the structural model are then imposed by these parameters. In a complementary and definitive experiment, they use Ca K-edge EXAFS studies to probe the binding site of the native cofactor for any nearby Mn, within {approx} 4 {angstrom}. This is analogous to the Sr EXAFS studies already published, but it focuses on the native cofactor and avoids the treatments involving Ca depletion and Sr substitution. The samples examined were PS II membrane particles from spinach. This new technique promises to be a more sensitive and direct probe of the calcium binding site in PS II than Sr EXAFS. Clarifying whether the Ca cofactor is proximate to the Mn cluster, and finding its coordination environment at the various intermediate S-states of the OEC will reveal its important role in oxygen evolution.« less